1. Field of the Invention
The present invention relates to a device which actuates a clutch pack in a limited slip differential and which uses a pressure regulating solenoid valve to control torque across the differential.
2. Description of Related Art
In conventional limited slip differential systems, torque transfer is controlled by a fluid bleed-off orifice. This generates a torque versus shaft speed (rpm) curve for each orifice open area and fluid viscosity. Since the flow will change with each viscosity change, the viscosity must be accurately modeled.
Some systems use a position-controlled valve to control flow open area. U.S. Pat. No. 6,413,182 B1 to Yates III et al., for example, discloses a limited slip differential having a thermal compensating valve for regulating torque bias. In this differential, a clutch piston is provided with flow control orifices permitting a flow of fluid from a clutch pressure chamber to a flow return circuit leading to an axle housing serving as a fluid reservoir. A thermal compensating valve is mounted on the clutch piston and is operable to decrease the flow area of the orifices upon increasing temperature and to increase the flow area upon decreasing temperature.
U.S. Pat. No. 6,183,387 B1 to Yoshioka discloses a variable pressure relief system for a hydraulically actuated limited slip differential including an annular actuator ring which is movable to adjust a maximum pressure attainable in a rotatable gear case.
U.S. Pat. Nos. 6,283,885 B1 to Irwin and 6,342,022 B1 to Sturm disclose pressure relief valves for hydraulically actuated clutch packs in limited slip differential or torque coupling devices.
Each of the four patents referred to above is generally related to the subject matter of the present invention, and the entire disclosure of each of these four patents is incorporated into the present application by reference.
It is one object of the invention to provide an actuation system for a limited slip differential in which the need to model predicted fluid viscosity and the resulting pressure at the clutch pack piston to compensate for changes in pressure fluid temperature is eliminated
It is another object of the invention to provide such an actuation system which improves regulation of output torque, permits removal of a thermocouple, and eliminates the need to develop viscosity curves.
According to the invention, these objects are achieved by way of a particular clutch pack actuation system which can compress the annular clutch pack of a limited slip differential. The system includes a pump which can provide a supply of pressurized fluid, an annular member provided between the pump and the annular clutch pack, and an annular piston which is slidable within the annular member. Together with the annular member, the annular piston defines a clutch pressure chamber adapted to receive the supply of pressurized fluid. A fluid return line, by which the pressurized fluid is discharged from the clutch pressure chamber, is provided, and a valve, through which the pressurized fluid is returned to a fluid reservoir, is disposed in the fluid return line. The valve includes an element which is electromagnetically actuated so as to produce a desired pressure in the clutch pressure chamber.
A process of regulating pressure in a clutch pressure chamber also forms part of the present invention.
By way of the invention, the differential does not require a thermocouple, and modeling of fluid viscosity is not required. In addition, torque transfer is more closely controlled, and regulating the differential preload is easier with a valve spring than by way of orifice size adjustment.